perf(daemon): skip the UI-thread datadir lock wait

Starting the daemon polls for an already-running node holding the
datadir, sleeping up to three 100 ms intervals to let a shutting-down
node clear out. When a start runs on the UI thread, that is up to 300 ms
of blocking sleep on the render thread.

- The lock-wait poll count is now a parameter. A start invoked on the UI
  thread passes 0, which does a single check and never sleeps; every
  other caller keeps the three-poll wait. The default is unchanged, so
  background starts behave exactly as before.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/daemon/embedded_daemon.h

@@ -61,7 +61,10 @@ public:
      * @return true if process started successfully (or is already running); false while a stop
      *         session (beginStop() .. endStop()) is still in progress
      */
-    bool start(const std::string& binary_path = "");
+    static constexpr int kDatadirLockWaitMaxPolls = 3;   // ~300ms total, breaks early on exit
+
+    /** @param lockWaitPolls max 100ms datadir-lock waits (0 = single check, never sleeps; for the UI thread) */
+    bool start(const std::string& binary_path = "", int lockWaitPolls = kDatadirLockWaitMaxPolls);
     
     /**
      * @brief Stop the daemon process this object launched (never any other dragonxd)